xplat RN: fix flow for useSyncExternalStore

Summary:
They flow type for useSyncExternalStore is incorrect and in consistent with www.

v1.0.0 of use-sync-external-store has the third parameter:
4997515b96/packages/use-sync-external-store/src/useSyncExternalStoreShimClient.js (L37)
This revision is 1.0.0:
4997515b96/packages/use-sync-external-store/package.json

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D44163470

fbshipit-source-id: 6582b20ba09f5840b444a1ca3d8fc12d6c783a00

packages/react-native/flow/use-sync-external-store.js

@@ -15,5 +15,6 @@ declare module 'use-sync-external-store/shim' {
   declare export function useSyncExternalStore<Value>(
     subscribe: (callback: Function) => () => void,
     getCurrentValue: () => Value,
+    getServerSnapshot?: () => Value,
   ): Value;
 }